This invention relates generally to a fabric or clothes dryer and more particularly concerns a vacuum-assisted dryer which can dry fabrics or clothes more quickly than conventional dryers.
Conventional household dryers, such as that described in U.S. Pat. No. 4,817,298, assigned to the same assignee as the present invention, include a drum for receiving clothes or other fabrics to be dried which is rotatable about a horizontal axis. During operation, the drum is rotated to tumble the fabrics while heated air is passed through the drum to extract moisture from the fabrics.
When a piece of fabric is laden with moisture, the water in the fabric is believed to exist in two different states, free moisture and bound moisture. Free moisture is moisture which is not held with any significant adhesive force in the fabric. In fact, as far as evaporation is concerned, the free moisture at the surface of the fabric behaves just like a free surface of water. Thus, the free moisture freely evaporates at the saturation pressure of water at the fabric temperature. On the other hand, bound moisture is held by relatively strong molecular forces. The net result of these forces is that the bound moisture is not maintained as a free layer of water at the surface of the fabric and hence does not freely evaporate at the saturation pressure. Bound moisture comprises about 25% of the mass of the wet fabric.
When wet fabrics are exposed to a warm flow of air in a conventional dryer, three periods of drying are discerned. The first period is a "warm up" period where the fabrics and their moisture content reach a steady drying temperature. During the second period, called the "constant rate" period, the free moisture is dried at a constant rate. After all the free moisture is depleted, the third or "falling rate" period begins. In this period, the bound moisture is removed at a decreasing drying rate. This drying cycle is a relatively lengthy process which takes longer than the washing cycle of domestic washing machines. This difference can create an inconvenience to persons doing multiple loads of laundry in that the washing machine will often be sitting idle holding wet clothes from a finished cycle because the dryer is still drying a previous load.